Optical or photonic links that comprise a laser, an optoelectronic modulator and a photodetector are known in the realm of RF photonics.
A prior art grating intensity modulator is described in an article by R. Kim et al. (Electronics Letters, vol. 41, no. 18, 1 Sep. 2005) and is illustrated in FIG. 1. This modulator has a bulk electrode in which the RF signal is applied to the electrode as a whole rather than being propagated along the electrode, from one end of the electrode to the other. This prior optical grating waveguide intensity modulator has the RF modulating signal applied to the entire length of the grating by means of a lumped element electrode. Thus, the capacitance of that electrode limits the frequency response of the modulator.
Another modulator is described U.S. Pat. No. 7,835,600 noted above and in an article by D. Yap et al. (Digest 2010 IEEE International Topical Meeting on Microwave Photonics, pp. 35-38). In that modulator, the electrode is part of an RF waveguide, to avoid limiting the frequency response by capacitance effects, and the RF and optical field propagate in perpendicular directions. The maximum modulation frequency can be limited by the group delay of the light propagating through the grating. For the prior arrangement of perpendicular propagating optical and RF fields, the group delay through the grating should be substantially less than one-half cycle of the highest frequency RF modulating signal. The modulation efficiency of these prior modulators, which describes the amount of intensity modulation that is obtained for a given modulation of the voltage applied at the electrodes, increases as the length of the grating is increased and as the size of the effective periodic refractive-index steps is increased. However, these increases in the grating length and index step also increase the group delay of the light. As a result, an improvement in the modulation efficiency would be achieved only at the expense of a corresponding reduction of the maximum modulation frequency and the modulation bandwidth.
Although there has been a long-standing desire to improve the modulation frequency achieved with optical grating intensity modulators, such a modulator has not been achieved in the past. This disclosure describes how such a high-frequency optical-grating modulation can be and has been achieved.
There is a need to obtain high efficiency intensity modulators that have large modulation bandwidths. A common prior art wideband intensity modulator is a Mach Zehnder interferometric modulator. This prior modulator (such as illustrated in FIG. 2a) contains an optical splitter (or a 3 dB coupler) that divides the input light into two optical-waveguide arms. The light in those two arms is then combined by an optical combiner (or another 3 dB coupler) to provide the output of the modulator. The intensity of the output light depends on the relative phases of the light coupled from the two waveguide arms, and the interference between that light.
To achieve a wideband modulation response, a goal of these prior Mach Zehnder modulators is to match the velocity of the traveling RF field with the velocity of the traveling optical field (the light being modulated). Some prior Mach Zehnder modulators also contain cascaded sets of multiple optical-waveguide gratings in the two arms of the interferometer. The desired function of the gratings in these prior modulators is to change the group velocity of the light propagating through them. The applied RF field changes the optical refractive index of the waveguide and grating material to produce a net change in the phase of the light propagated through the chain of grating reflectors and waveguide segments without substantially changing the intensity of that light. To produce a change in the intensity of the light output from the modulator, that chain of grating reflectors and waveguide segments is incorporated in the two arms of a Mach Zehnder interferometer and receive different modulation of their refractive index, resulting in a phase modulation of the light propagating through those arms. Any direct intensity modulation of the light directly by the gratings of these prior art modulators is undesirable and would interfere with the desired operation of the Mach Zehnder modulators.
Optical Mach-Zehnder modulators that contain multiple gratings in their interferometer arms wherein the overall transmittance of the modulator is modulated by the electric field carried by a traveling-wave RF electrode are described in articles by Shaw et al. (Electronics Letters, vol. 35, no. 18, 2 Sep. 1999, pp. 1557-1558), by Taylor (Journal of Lightwave Technology, vol. 17, no. 10, October 1999, pp. 1875-1883) and by Khurgin et al. (Optics Letters, vol. 25, 2000, pp. 70-72). FIG. 2b shows an illustration of a portion of these prior modulators. In some modulators, the multiple gratings are separated by optical-waveguide sections. The function of the multiple gratings is to serve as optical reflectors, with each pair of such grating reflectors and the optical waveguide segment between them acting as an optical etalon. In others of these modulators, multiple pairs of grating segments that have differing grating periods are cascaded end to end. The cascade of etalons or cascade of pairs of dissimilar gratings slows down the group velocity of the light propagating through that cascade.
In these prior art modulators, the grating is used solely as a means to increase the group delay (or decrease the group velocity) of light propagating through the grating. The prior art modulators that have multiple grating reflectors in a cascade of Fabry Perot resonators operate at a wavelength for which the grating transmittance is minimal (transmittance notch) and the grating reflectance is high. The cascade of resonators produces the “slowing” of the velocity of the light. The prior cascade of multiple pairs of grating segments that have different grating periods likewise produces a “slowing” of the velocity of the light. For modulators formed from lithium niobate material, this “slowing” of the light makes it easier to match the velocity of the co-propagating RF field traveling in the RF electrodes with the velocity of the light in the two arms of the interferometer.
Although some prior modulators have combined optical waveguide gratings with RF traveling-wave electrodes in which the RF field co-propagates with the light traveling through the optical waveguide gratings, the desired effect of the RF field is to produce only a phase modulation of the light exiting the grating and not to produce any intensity modulation of the light transmitted through a given optical-waveguide grating or cascade of grating segments.